1. Field of the Invention
The present invention relates to a network system for mobile radio communication in which a network system for mobile radio terminal is connected to a communication through a public or private radio base station to perform data communication with a server unit, and more particularly, to a network system for mobile radio communication which can prevent data losses even if slot errors or a drop in electric field strength occur between a mobile radio terminal and a public or private radio base station.
2. Description of the Prior Art
As shown in FIG. 1, a conventional network system for mobile radio communication is constituted by a radio connection unit 55 for selecting a line to be connected through a communication network 56, radio base stations 53 and 54 respectively installed in a plurality of unit cells, and a mobile radio terminal (to be referred to as a PS hereinafter) 48 which establishes a radio channel with a radio base station while moving. The PS 48 includes a buffer memory (to be referred to as a BM hereinafter) 50 for temporarily storing data in a line switching operation, a data device 51, a data processing apparatus 49, and a communication controller 52.
As a data transmission scheme in such a system, the scheme disclosed in Japanese Unexamined Patent Publication No. 59-212040 is available.
More specifically, when the radio base station (1) 53 determines the necessity for line switching during communication between the PS 48 and the radio base station (1) 53, the radio base station (1) 53 sends a line switching notification to the radio connection unit 55, and also sends a communication interruption notification to the data processing apparatus 49. Upon reception of the notification, the data processing apparatus 49 transmits an acknowledgement signal to the radio connection unit 55 and starts to store, in the BM 50, the communication data which has been directly transmitted. When the radio connection unit 55 receives the acknowledgement signal from the data processing apparatus 49, the communication between the PS 48 and the radio base station (1) 53 is interrupted. While the data processing apparatus 49 is storing the communication data, line switching processing is performed. When the radio base station (2) 54 to serve next is determined, the radio connection unit 55 sends a switching instruction to the radio base station (1) 53, that has been serving up until now, and the radio base station (2) 54 that is to serve next. Upon reception of the switching instruction, the radio base station (1) 53 as the currently serving station sends a switching instruction to the communication controller 52. When these operations are complete, the communication is resumed. When the communication is resumed, the data processing apparatus 49 reads out the data stored in the BM 50 and transmits the data to the radio base station (2) 54 through the communication controller 52. When all the data are read out from the BM 50, direct communication without the mediacy of the BM 50 is started again.
The following problems are posed in the above conventional data transmission scheme.
First, in data transmission between a server unit, which is assumed to be outside the area covered by a communication network, and a PS, the server unit that transmits/receives data to/from the PS has no BM for storing data. For this reason, if a hit occurs during data transmission from the server unit to the PS, data is lost.
Second, no consideration is given to a case wherein line disconnection cannot be avoided because of continuous slot errors or a continuous reduction in electric field strength. For this reason, data is lost when the line is disconnected.
Third, the radio base station must monitor the reception state of the PS so as to send a line switching instruction thereto. For this reason, line switching processing is complicated, resulting in a decrease in line switching speed.